1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and more particularly to an image forming apparatus and an image forming method using an intermediate transfer method.
2. Discussion of the Background
An intermediate transfer method is widely used in image forming apparatuses. An example intermediate transfer image forming apparatus includes a photoreceptor, an image forming mechanism, an intermediate transfer belt, an intermediate transfer roller, and a secondary transferer. The intermediate transfer belt is an endless belt that may move in synchronization with the photoreceptor.
After the surface of the photoreceptor is charged, an electrostatic latent image is formed on the photoreceptor. The electrostatic latent image is developed with a toner. The intermediate transfer roller is pressed to the photoreceptor via the intermediate transfer belt to form a transfer nip. The intermediate transfer roller is configured to apply a transfer bias to the transfer nip to transfer the toner images from the photoreceptor onto the intermediate transfer belt. The secondary transferer transfers the image onto a transfer medium.
Recently, the use of low melting toner has been increasing to meet the demand of energy saving and resource saving. Generally, an additive (e.g., inorganic fine particle) is added to the surface of a toner mother particle. However, the additive may be embedded due to mechanical stress, for example when the toner is agitated in a developing unit. The rate of additive being embedded into the mother particle (additive burial rate) is higher in the case of low melting toner. The low melting toner generally has a burial rate of 40 percent or greater.
If the above burial rate is 40 percent or greater, the additive tends to be embedded into the mother particle early and fluidity of the toner may decrease.
When the fluidity is decreased, it is difficult to charge the toner to a predetermined potential by agitating the toner in the developing unit. Further, the toner tends to have an increased agglomerating property. Therefore, the toner may easily agglomerate and have greater mechanical adhesion to the photoreceptor when receiving pressure at the transfer nip. It may become difficult to electrostatically transfer the toner to the intermediate transfer belt and transfer performance of the toner is decreased.
The transfer performance may be enhanced by increasing a transfer bias to increase an electrostatic force. However, when the transfer bias is increased, a potential may be injected into the toner and the toner may be reversely charged. The reversely charged toner may return to the photoreceptor at an exit of the transfer nip.
Further, in a tandem image forming apparatus, in which a toner image on an intermediate transfer belt passes though a plurality of transfer nips, the reversely charged toner may be transferred onto a photoreceptor of a different color (reverse transfer). If such reverse transfer occurs, the toner in a resulting image is partly insufficient.
A technology to lower pressure at an intermediate transfer nip by using an elastic intermediate transfer belt has been proposed. Further, using a brush as a bias applicator in an image forming apparatus has been proposed.